2022 Volume 1

Optimizing the fibre property distribution could increase the pulp properties as well as the process efficiency of chemimechanical pulps (CMP/CTMP). This can only be achieved with a better understanding of how evenly distributed sulphonate concentrations are between the individual CTMP fibres. Given that the quality of wood chips varies with the chipping methods used in pulpwood processing and sawmill processing, as well as with the chip screening system, it is a challenge to develop an impregnation process that ensures even distribution of sodium sulphite (Na2SO3) in the liquid used to impregnate the Chemimechanical pulp (CMP/CTMP). Therefore, the distribution of sulphonate groups within wood chips and individual fibres must be measured at the microscale level. On a micro level, the degree of unevenness, i.e., the amount of fibre sulphonation and softening before defibration, cannot be determined due to the use of excessively robust or complex processing methods. By having it, we could better understand how sulphonation occurs before defibration, so we could improve impregnation. Developing a laboratory scale miniaturized energy dispersive X-ray fluorescence (ED-XRF) method that measures sulphur distribution at the fibre level can enable us to study the influence of impregnation improving processes.

The export of market kraft pulp is a significant part of the Canadian forest products industry. Although northern softwood kraft pulps are premium products in the international pulp marketplace, there is interest in producing truly specialty pulps whose properties extend beyond the physical and chemical property boundaries of current pulping and bleaching operations. Whereas the pulping and bleaching literature has for decades focused on improving pulp properties, we know of only a few examples of post-bleaching fiber modification in pulp mills. Instead, the pulp producers leave it to papermakers to tune paper properties with chemical additives in the papermaking processes. Most papermill fiber chemical treatments including sizing additives, dry strength resins, and wet strength resins, involve interactions with the exterior surfaces of pulp fibers. We propose that market pulp mills producing dry, or nearly dry, pulp offer a unique opportunity to influence fiber surface properties by fixing reactive polymers onto fiber surfaces when the pulp is heated on pulp drying machines. The objective of the work described herein was to develop new approaches to modify pulp fiber surfaces at the end of the pulp mill bleaching processes through polymer grafting. This contribution covers the highlights of recent publications [1-4].

Cellulose nanofibril (CNF) foam, which has advantages of sustainability and biodegradability, has a potential to apply to diverse fields including packaging, thermal insulation, and absorbent. In recent days, the oven drying of Pickering-stabilized CNF wet foam was proposed as an alternate approach to manufacture CNF-based porous foams. To produce CNF foam with uniform structure, the properties of wet foam are very important. In this study, carboxymethylated cellulose nanofibril (CMCNF) was used to prepare CNF foam. The effects of wet foaming conditions such as CMCNF consistency, surfactant amount, and shear rate on the properties of the wet and dry foams were investigated. A low addition level of surfactant led to insufficient generation of bubbles, whereas high levels of surfactant generated unstable wet foam. A proper amount of the surfactant and CMCNF consistency yielded wet foams with excellent stability. CNF wet foam with high stability resulted in CNF foam with uniform pore structure and high compressive strength. The shear rate during wet foam generation also had a significant impact on the foamability of wet foam, which determined the density and the pore size of the oven-dried CNF foam.

Foam forming of cellulose fibre materials is based on an interaction between fibres and bubbles, which can take several material properties to new levels. To control the formed structure, the mechanisms of this interaction have been systematically investigated. This started with captive bubble studies where we analysed the interaction of a single bubble with various smooth cellulose and silica model surfaces. The bubbles adhered only to hydrophobic surfaces, and this attraction was sensitive to the surface tension. From this simplest case, the studied system gradually became more complex. We found that a bubble adheres weakly also to a submerged cellulose nanofibre (CNF) film, which could be explained by nanoscale surface roughness capturing nanobubbles. The interaction with real fibres was studied by pressing a single bubble against a fibre bed in water and sodium dodecyl sulphate (SDS) solution. Fibre type and surface tension had all apparent effects on the attachment. In the case of natural fibres, the presence of hydrophobic lignin clearly increased the fibre attachment on a bubble, while added SDS decreased the attachment with all fibre types. These findings agreed with the mechanisms found earlier using the model surfaces. Finally, when forming thick nonwoven materials using hydrophilic and hydrophobic viscose fibres, differences in fibre network structure and strength properties depended on the fibre hydrophobicity and surfactant type, as suggested by the results obtained in simpler systems.

The molecular mechanisms behind the interactions between fibres in fibrous networks, and their link to paper/network strength, have long been under intense scientific investigations and scientific debate (Wågberg and Annergren 1997, Lindström et al. 2005; Hirn and Schennach 2017) but still there is no unified view on how the strength of fibre/fibre joints and network strength can be linked to different molecular mechanisms (Wohlert et al. 2022). Historically the interaction between cellulose-rich fibre surfaces was ascribed to hydrogen bonding and elaborate models were developed for linking mechanical properties of fibrous networks to the H-bonding between the surfaces (Nissan and Batten Jr 1997). This is however an oversimplification for several reasons. First of all, the H-bonds are very specific, which means that they are short-ranged and not implicitly additive over the material volumes engaged in the contact region between the fibres. Secondly the molecular interactions in the fibre/fibre contact zones are actively participating in the formation of the fibre joints, i.e. in the making of the fibre/fibre joints. The interactions in the wet state will hence affect the dimensions of the wet and dry contact zones and they will also create built-in stresses in the zones of contact in the dried fibre/fibre joint. The dry mechanical properties of the fibrous networks, i.e. the breaking of the joints and the fibres, will then be controlled by the molecular contact zone, the interactions in the contact zone, and also outside the molecular contact zone but within the molecular interactions range, the number of molecular contacts/network volume and the individual fibre strength.

In this work we visualize the evolution of the 3D microstructure of a mono-disperse nylon fibre floc undergoing uniaxial compression. In total seven stages of compression were visualized using X-ray microcomputed tomography. We observe that at the early stages of compression, densification occurred through sliding and rotation; at later stages, densification occurred though individual fibre deformation. We quantified these images and estimated the density of interparticle contacts ρ_c as a function of the compressive strain ε_c and found that ρ_c ∝ ε³_c

We have studied ways of improving strength properties of paper made from high yield pulps and lignin-rich chemical pulps by utilizing the thermoplastic properties of the lignin present in the fibre walls. Both dry and wet strength can be improved by hot pressing of sheets made from lignin-rich pulps. In this paper, we focus on aspects of the wet-strength development as a function of lignin content and temperature. Here we apply an activation energy evaluation approach to study lignin intermixing or interdiffusion. By means of hot pressing, it is possible to reach wet strength levels up to 50% of the dry strength level, provided that we use pulps with high enough lignin content. Our study included hot pressing of high yield pulps such as thermomechanical pulp (TMP), chemithermomechanical pulp (CTMP), high-temperature chemithermomechanical pulps (HTCTMP), unbleached northern softwood kraft (NSK) and northern bleached softwood kraft (NBSK). The sheet pressing trials were performed for varied temperatures from room temperature up to 270°C. As the activation energy for the high yield pulps and the lignin-rich NSK were all in the range of 20-32 kJ/mol, we suggest that the wet strength development as function of temperature has a similar mechanism as long as the pulp fibres contain enough lignin. We also suggest that the phenomenon involves intermixing and/or interdiffusion of wood polymers between adjacent fibres when they are in a close contact. Most probably both the amorphous wood polymers, i.e. the linear hemicelluloses and the cross-linked lignin, mix with each other across the fibre-fibre or even more probable over the fibril-fibril contact surface. While the hemicellulose can intermix already at room temperature under moist conditions, the lignin intermixes more easily at the higher temperature we use. We do not know how far the hemicellulose or lignin could move within the fibre walls, but it seems that the amount of lignin present on the fibre surfaces plays an important role.

The physico-chemical characteristics of wet fibre surfaces and their role in fibre bonding and paper properties have been under debate and research for decades. The gel-likeness of the fibre surfaces has been addressed in many studies but has not been explicitly demonstrated. In this study, the structure of wet beaten kraft pulp fibre surface and its similarity with microfibrillated cellulose (CMF) was shown using helium ion microscopy (HIM) imaging. Beaten kraft fibres and CMF were dried using two mild drying methods to preserve the delicate fibrillated structures. The fibre surface had a strong resemblance with gel-like CMF material. The amount of external fibrillation varied along the fibre length and was often shown to extend tens of micrometers from the fibre surface. The gel-like behaviour of wet fibrillated material was demonstrated using rheological tests. The examined CMF and CNF samples with solids contents of 1.97% and 1.06%, respectively, showed gel-like behaviour. A low gelling point suggests that fibrillated fibre surfaces have the ability to transfer forces at very low consistencies, and that the ability increases as a power function of the solids content. This is assumed to play a remarkable role in increasing inter-fibre adhesion and in transmitting inter-fibre forces within consolidating webs, whether arising from external tensions or internal drying stresses.

Apart from its wide application in the paper, textile and biomedical industry, cellulose is now an emerging alternative reinforcement to improve the properties of polymers. Numerous research focuses on the development of renewable nanocomposites. In this context, nanocellulose liberated from plant cell walls or produced by bacterial serves as excellent candidate due to its inherently nano-sized nature, high crystallinity and high Young’s modulus. However, numerous cellulose-reinforced polymer nanocomposites reported in literature often failed to fully exploit the fibril tensile stiffness and strength, estimated to be 114 GPa and >2000 MPa, respectively. Nanofibrils can be compounded directly into polymers as reinforcement or used in paper form to produce laminated paperbased composites.

In recent years the application of paper for packaging has been growing steadily. Food packaging underlies special scrutiny, as the package has to prevent the escape of aroma molecules at the one hand and prevent the contamination of the packaged good with unwanted substances on the other hand. For the transfer of organic molecules from the packaging to the food and vice versa one has to look at volatile organic molecules. For transport of such molecules through the porous structure of paper the interaction of the molecules with the surface of the paper fibers plays an important role.